During development, sensory innervation to taste buds is tightly regulated by the actions of two neurotrophins, brain-derived neurotrophic factor (BDNF) and neurotrophin-4 (NT4). Although both of these factors regulate gustatory innervation, their specific functions are different from one another. BDNF is expressed in gustatory epithelium and determines whether gustatory neurons will locate and innervate taste placodes during development, while NT4 does not. Both BDNF and NT4 regulate gustatory neuron and taste bud number, but they have their influences at different times during development. The long range goal of this research is to understand the cellular and molecular mechanisms that regulate the neural innervation to and the maintenance of taste buds during development. The proposed studies focus on mechanisms by which these two neurotrophins, BDNF and NT4, can influence various aspects of neural innervation through the same receptors, TrkB and p75. The proposed studies use cell counting, tract tracing, immunohistochemistry and scanning electron microscopy in gene knockout mice to determine when TrkB and p75 are required for specific aspects of gustatory development, including axonal extension into the tongue, target selection, and geniculate neuron/taste bud survival. Furthermore, these studies combine these anatomical approaches with sophisticated genetic tools allowing the function of single phosphorylation sites on the TrkB receptor protein to be blocked in vivo. These experiments will determine which TrkB intracellular signaling pathways are required for axonal extension into the tongue, target selection, and geniculate neuron/taste bud survival. Together these studies test the hypotheses that TrkB mediates the effects of both BDNF and NT4, but that each function of these neurotrophins requires a different intracellular signaling pathway. While p75's, more subtle role, is to modulate the regulation of gustatory development by neurotrophins. Because these experiments examine the fundamental mechanisms of how neurotrophins regulate neuronal survival and axon targeting via Trk receptors, this project has important implications for the therapeutic use of these signaling mechanisms in controlling neuron survival and axon targeting during regeneration. PUBLIC HEALTH RELEVANCE: The taste system is an ideal and commonly used system for examining axon regeneration following injury. We are examining the signaling mechanisms for two factors (neurotrophins) that influence axon targeting and neuron survival during development. By understanding of how these signaling mechanisms normally function, techniques can be developed to manipulate these important signaling factors during nerve regeneration which will improve reinnervation and function following nervous system injury.